1. Field of the Invention
The present invention relates to a low-pass filter which is applied to a high-frequency circuit, and more particularly, it relates to a low-pass filter employing a multilayered chip which is formed by stacking a plurality of dielectric layers on each other.
2. Description of the Background Art
A well-known low-pass filter is described with reference to FIGS. 5 and 6. FIG. 5 is an exploded perspective view showing a conventional low-pass filter 130, and FIG. 6 is a perspective view showing the appearance of this low-pass filter 130.
The low-pass filter 130 is formed of a multilayered chip, which is obtained by stacking dielectric substrates 103, 107, 110 and 111 and a dielectric sheet 112 on each other.
The dielectric substrate 103 is provided on its upper surface with microstriplines 101 and 102, which are in the form of folded straight lines. The dielectric substrate 107 is provided on its upper surface with capacitor electrodes 104 to 106. The dielectric substrates 103 and 107 are stacked between the dielectric substrates 110 and 111.
The dielectric substrates 110 and 111 are provided on upper surfaces thereof with ground electrodes 108 and 109 respectively.
This multilayered chip can be obtained by forming the aforementioned electrode structures on dielectric green sheets, thereafter stacking the green sheets on each other, and integrally firing the ceramic material on the electrode material. Alternatively, fired dielectric substrates may be used for the dielectric substrates 103, 107, 110 and 111 respectively.
External electrodes 113 to 116 are formed on first longer side surfaces of the dielectric substrates 103, 107, 110 and 111, respectively, and the dielectric sheet 112, while external electrodes 117 to 120 are formed on second side surfaces thereof respectively. As clearly understood from FIG. 6, these external electrodes 113 to 120 are formed on longer side surfaces of a multilayered chip 131 as assembled, while FIG. 5 shows the same in an exploded view.
As clearly understood from FIGS. 5 and 6, the ends of the microstripline 101 are electrically connected with the external electrodes 114 and 117 respectively. The ends of the external electrode 102 are electrically connected with the external electrodes 115 and 120 respectively. The capacitor electrode 104 is electrically connected with the external electrode 117, while the capacitor electrode 105 is electrically connected with the external electrode 120. Further, the capacitor electrode 106 is electrically connected with the external electrodes 114 and 115. In addition, the ground electrode 108 is electrically connected with the external electrodes 113, 116, 118 and 119, while the ground electrode 109 is also electrically connected with the external electrodes 113, 116, 118 and 119.
The microstriplines 101 and 102 are adapted to form inductance components, i.e., inductances L1 and L2, respectively, appearing in an equivalent circuit shown in FIG. 7.
Further, the capacitor electrodes 104 to 106 along with the ground electrode 109 form capacitors C1 to C3, respectively, appearing in the equivalent circuit shown in FIG. 7.
The dielectric sheet 112 is adapted to protect the ground electrode 108 which is provided on the dielectric substrate 110.
In this low-pass filter 130, the external electrodes 117 and 120 are employed as input and output leads respectively. Further, the external electrodes 113, 116, 118 and 119 are connected with the ground potential. The external electrode 114 is adapted to electrically connect the end of the microstripline 101 with the capacitor electrode 106. Similarly, the external electrode 115 is adapted to electrically connect an end of the microstripline 102 with the capacitor electrode 106.
When the external electrodes 117 and 120 are employed as input and output leads respectively and the external electrodes 113, 116, 118 and 119 are employed as portions connected to the ground potential, the low-pass filter 130 operates in accordance with the equivalent circuit shown in FIG. 7.
FIG. 8 shows the transmission characteristic of the low-pass filter 130. As clearly understood from FIG. 8, insertion loss is damped at the resonance frequency, and thereafter decreased in the frequency band above the resonance frequency. Such decrease of the insertion loss in the high frequency band occurs because high-frequency signals bypass the microstriplines 101 and 102 via floating capacitances which are developed across the microstriplines 101 and 102 respectively. Consequently, it is difficult to attenuate an unnecessary high-frequency signals.
In order to shift the passband of the low-pass filter 130 toward the lower frequencies, the resonance frequency of the low-pass filter 130 may be reduced. In order to reduce the resonance frequency, however, it is necessary to increase the lengths of the microstriplines 101 and 102 thereby increasing the values of the inductances L1 and L2, or to increase the areas of the capacitor electrodes 104 and 106 thereby increasing the electrostatic capacitances of the capacitors C1 to C3. In this case, the low-pass filter 130 is disadvantageously increased in size.